Throughout all of the industrial sectors of the world many mechanical devices are subjected to wear caused by abrasion, erosion, and/or corrosion during their normal service life. Billions of dollars are spent by industry to replace components which fail prematurely because of excessive wear in inert and corrosive service environments. Many parts may be made to last longer if they were manufactured from harder, corrosive resistant materials however, the cost of doing so is often prohibited and can mean the difference between a successful operation and a unsuccessful operation because of excessive costs.
A number of methods are available for surface hardening or depositing corrosion and wear resistant materials on industrial parts. The oldest known methods are diffusion treatments, nitriding and carburizing of ferrous based materials. The disadvantage in using these techniques is that they involve subjecting the parts to elevated temperatures. Apart from the high costs associated with the energy and operation time, subjecting a part to elevated temperatures can cause size changes and loss of mechanical properties which would render the part unsuitable for use and/or would require a further heat treating operation and a subsequent cleaning operation to be performed after the surface treatment.
Electroplating, most commonly used to produce hard chromium or nickel coatings, involves cleaning the parts to be coated to a high degree and involves toxic solutions which are costly when disposed of in an environmentally safe manner.
Chemical and physical vapor deposition of coatings require high capital investment, high processing costs and are limited to very thin coatings and small parts. Of the thermal spray deposition methods which can be used to coat parts of unlimited size with coatings of unlimited thickness flame spraying often yields a porous coating with oxide inclusions.
Plasma spraying especially if performed in a vacuum or atmosphere chamber, will yield dense homogeneous coatings but is expensive and therefore limited in use.
High velocity detonation guns can deposit dense ceramic coatings on substrates but the equipment, feed powders and processing are very expensive.
Electric arc spraying with inert gases can produce dense, homogeneous coatings which bond well to a variety of substrate materials. Arc-sprayed titanium nitride which does not require high enthalpy flame is a cold process compared to the high heat input plasma and flame spray processes which can damage or distort the substrate material. Furthermore, the capital equipment and operating costs are less than one-half that of the plasma high velocity spraying methods and about order of magnitude less than that of the chemical vapor deposition. In electric arc spray of titanium nitride type coatings, disclosed in this invention, the surface to be coated requires no special preparation other than grit blasting.